ar71xx/mikrotik: disable unused ethernet switch and phy drivers

Disable the drivers for the following ethernet switches:
  Realtek RTL8306
  Realtek RTL8366/8367
  Marvell 88E6060 (DSA)
  Marvell 88E6063 (DSA)

Also disable the phy driver for Marvell PHYs.

None of the supported RouterBOARDs are using any device
which needs these drivers.

Disable the DSA framework too, because it is not required
by the remaining switch drivers.

This reduces the compressed kernel size by ~20KB.

Signed-off-by: Gabor Juhos <juhosg@freemail.hu>